1. Field of the Invention
The present invention relates to configurations of digital transmission apparatuses and configurations of communication networks using the digital transmission apparatuses, and more particularly to configurations of transmission apparatuses for use in a synchronous digital hierarchy and configurations of communication networks using the multiplex transmission apparatuses.
2. Description of the Related Art
In today's digital transmission networks, the technology of synchronization has been advanced to such a degree that a communication network is synchronized with faster transmission apparatuses employing optical transmission. For functions and configurations of the digital transmission networks and transmission apparatuses, worldwide standards have been established such that a transmission apparatus and/or a communication network may be introduced in conformity to the standards to provide high quality transmission anywhere in the world. Examples of specific standards may include the standard (established in 1988) on a transmission system referred to as “SDH” (Synchronous Digital Hierarchy) defined in Recommendation G. 707 and so on by International Telecommunication Union (hereinafter abbreviated as “ITU-T), and the standard (established in 1991) on a transmission system referred to as “SONET” (Synchronous Optical Network) defined in Standard T1.105 by American National Standard Institute (hereinafter abbreviated as “ANSI”), both of which define the configuration of optical synchronous communication systems and functions of transmission apparatuses.
Both SDH and SONET define the processing (transmission, multiplexing/demultiplexing, and so on) of a synchronous multiplexed signal (frame) which comprises a main signal portion referred to as a “Payload” in which digitized main signals are multiplexed, and signals referred to as “overheads”, added to the payload, for administration and maintenance operations for a transmission apparatus and communication network. The overheads include pointers which are used to perform stuff controls such as frame phase synchronization and frequency adjustment to provide a transmission system which has a less transmission delay and a higher administration and maintenance operation performance than conventional digital synchronous transmission apparatuses. The overheads added to the frame are classified into a section overhead (SOH) and a line overhead (LOH). The section overhead is used for administration and maintenance operations for each transmission span between transmission apparatuses and regenerators (defined as a section), and generated in an apparatus (including a regenerator), transmitted through a transmission span, and terminated at a next apparatus. The line overhead is used for administration and maintenance operations for each transmission interval between transmission apparatuses which process multiplexed main signals (defined as a line). The line overhead is generated in a transmission apparatus, transmitted through transmission spans and regenerators, and terminated at a next multiplexing apparatus. Examples of transmission apparatuses and networks adopting the above-mentioned SDH or SONET are described in JP-A-4-79628 and JP-A-5-114892.
In a transmission network in conformity to SDH or SONET, multiplexers may be occasionally connected to each other on a transmission line through regenerators. For carrying out the administration and maintenance operations between the multiplexers in such a transmission network, the line overhead is used to transmit and receive data and speech signals necessary to the administration and maintenance operations between the multiplexers. For example, D-bytes referred to as “data communication channels” and E-bytes referred to as “orderwires” of the line overhead are used to transmit and receive such data and speech signals between the multiplexers. Specifically, when a multiplexer on the transmission side inserts data and speech information into D4-D12 bytes and E2 byte of a line overhead and transmits the line overhead onto a transmission line, the line overhead is terminated at a destination multiplexer through the transmission line and regenerators, thereby carrying out the administration and maintenance operations between the multiplexers.
As the number of subscribers increases in the transmission network or as an increased amount of signals is communicated through the transmission network, extension and/or modifications in the transmission network, such as installation of additional multiplexers and replacement to the transmission network to a faster transmission line, may be required for supporting the increase in subscribers and the amount of communicated signals. For example, if the amount of communications increases, the existing transmission network is reconfigured, wherein similar multiplexers to those so far used are additionally installed in the transmission network, and a plurality of faster and larger-scaled multiplexers are introduced for processing signals in place of regenerators to modify the transmission network so that the multiplexers are connected through faster transmission line in the transmission network.
However, the reconfiguration of the transmission networks as mentioned above results in a problem involved in SDH and SONET standards. Specifically, since the line overhead is terminated at each transmitter according to SDH and SONET standards, an overhead outputted from an existing multiplexer may be terminated at an additionally installed faster multiplexer, so that the overhead is not transmitted to a far-end multiplexer which has so far received this overhead. In other words, the reconfiguration results in a lack of the administration and maintenance operations previously performed between the multiplexers before the reconfiguration. Thus, while the transmission capability of the transmission network for transmitting main signals is improved by the reconfiguration of the transmission network, the reconfiguration causes changes in the administration and maintenance operation capability of the transmission network, such as lack of the administration and maintenance operations so far performed between transmission apparatuses. Since administration and maintenance operation information previously provided is no longer available to a craft person dedicated to the maintenance of the transmission network, the craft person may suffer from quite inconvenient situations.